The need for a new and more versatile personnel inspection system in mass transportation centers has increased in recent years. Traditional inspection systems such as metal detectors and X-ray imaging systems, although capable of near real-time detection, have limitations and adverse effects in the detection of concealed targets. Limitations of metal detectors include the inabilities to (a) provide precise target location, (b) detect plastic concealed weapons, and (c) detect certain metals because of sensitivity variation for various metals. Limitations of X-ray imaging of personnel include radiological health effects. Consequently, holography has been under investigation as an alternative or complementary approach to personnel inspection.
Application of holography to the problem of personnel surveillance has been limited because of the inability to either (a) produce an image of sufficient resolution, or (b) produce an image in real-time, or (c) a combination of both. It is recognized that use of millimeter wave electromagnetic radiation is not a physiological health hazard and such radiation penetrates certain materials, including but not only clothing.
U.S. Pat. No. 5,455,590 (Collins et al.) published Oct. 3, 1995 entitled “Real-time holographic surveillance system” whose contents are incorporated herein by reference discloses a holographic apparatus for near real-time imaging of a target. The apparatus utilizes millimeter wave radiation having a frequency from about 1 to about 110 GHz and comprises:
(a) a holographic array having a plurality of low-gain, end-fire antenna units spaced apart from about 0.25 to about 1.5 wavelength, wherein each unit both sends and receives millimeter wave radiation. The units are connected by a plurality of electronic millimeter wave switches permitting sequential operation of the units, the array spaced apart from the target with a low f-number;
(b) a holographic transceiver system for operating the units and providing each unit with millimeter wave radiation source, then receiving high frequency millimeter wave radiation reflection from the target and collected by the unit, then making an analog oscillated reference signal, together with an analog reflected target signal;
(c) a real-to-imaginary converter for converting the analog oscillated reference signal and the analog reflected target signal to an analog real part of a hologram and an analog imaginary part of the hologram;
(d) an analog to digital converter for converting the analog real part and the analog imaginary part to corresponding digital parts; and
(e) a computer for applying a backward wave propagation algorithm that preserves the low f-number to the digital real and digital imaginary parts of the hologram to reconstruct a holographic image.
The apparatus disclosed by U.S. Pat. No. 5,455,590 requires a plurality of low-gain, edge-emitting antenna (known as end-fire) units. Furthermore, operational amplifiers are required to increase the gain of the RF in-phase “I” and quadrature “Q” signals, the gains of the operational amplifiers being set to match the maximum range of the analog to digital (A/D) converters. The antenna is scanned in an X-Y scanner to fill the aperture. Scanning may be mechanical or electrical. Electronic scanning is provided by transmitting and receiving a signal from an individual antenna unit in sequence one at a time in successive order. Electronic millimeter wave switches are used to direct signals to the antenna units.
It would therefore be desirable to provide an improved holographic surveillance system wherein at least some of the above-mentioned drawbacks are reduced or eliminated. Specifically, it would be an advantage to provide holographic surveillance system that permit direct detection with no need for RF amplification or mechanical scanning, thus achieving lower cost and faster imaging rate.